Is 15 days enough to visit all four Hawaiian islands ?

You'll probably be too late for whale watching but may catch a straggler or two. Dolphins will still be around. You can technically "do" those 4 islands in that time, but you might end up exhausted and without much rest time if you're diving, etc. too. You're also not leaving much time for Oahu where there's the most things to do. Yes you can do a rental car, but parking is bad, or you can take thebus.